Building a guitar amplifier

Thread Starter

drk

Joined Mar 8, 2008
41
You mean something like in the picture (attached)? I've seen some devices like that.

I don't think the heatsink I got is that big, but isn't a heatsink like that much more than what the datasheet says is required?
 

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Audioguru

Joined Dec 20, 2007
11,249
Heatsinks have a rated "thermal resistance" that is shown in their catalogs online.. It is easy to calculate how much thermal resistance you need depending on the amout of maximum output power of your amplifier IC and its thermal spec's.
 

THE_RB

Joined Feb 11, 2008
5,438
Yep, but I would add never to skimp on a guitar amp heatsink! In guitar use the amp will often be pushed hard with wall to wall almost square wave full volume waveform, it's nothing like a typical audio amp used for playing music.

DRK, yep that diagram is exactly what I would suggest. Have a look at heatsink prices in the stores or on ebay, they are not that expensive and nice large $10 or $15 heatsink will make sure your amp gives years of reliable guitar use.

And for any thermal specs the amp chip has, there is a still an exponential curve for the time before failure based on heat. So running the amp chip even 5 degrees cooler will give a much longer life. Commercial products will skimp on amp chip heatsinks to save a buck but there is no need for you to skimp, it's better to build something to last for many years of hard work.
 

Audioguru

Joined Dec 20, 2007
11,249
Yep, but I would add never to skimp on a guitar amp heatsink! In guitar use the amp will often be pushed hard with wall to wall almost square wave full volume waveform, it's nothing like a typical audio amp used for playing music.
No.
When a class-AB amplifier is clipping like mad producing squarewaves, it is much cooler than at its rated power. The total output power when producing squarewaves is double the rated power because the power from the harmonics equals the power of the fundamental.

When producing squarewaves the output transistors switch on and off and do not spead time heating as a linear amplifier.

I agree that electric guitar blasting sounds are not music, they are noises.
 

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THE_RB

Joined Feb 11, 2008
5,438
Yep I get that AG and am not arguing with you about dissipation being higher when not clipping.

But guitar waveform does not necessarily clip the amp at max but it will be constantly working at some high volume (high power ouptut level) and they DO have a tendency to cook amp chips much more than the same amp chip in an audio HiFi style application.

For example if the amp supply is +/- 35v and it is outputting a constant +/-20v squarewave guitar "noise" into the speaker that will dissipate a lot of heat in the amp chip, compared to HiFi use.
 

Audioguru

Joined Dec 20, 2007
11,249
Aren't all 20W guitar amps turned up to 40W or more so the output clips like crazy? They call it "fuzz".
Then the output transistors are simply on and off switches and are nice and cool.

I wonder what an electric "geetar" sounds like when played through a hifi system that is wideband and has no distortion. Has anybody ever tried it?
 

#12

Joined Nov 30, 2010
18,222
Yes. The single tones (nearly pure sine waves) that come out the speakers are so simple that they seem to be alone in a huge room. They seem thin and weak with no overtones. It's like asking a small band to put down their instruments and have one person whistle.

I'm thinking you need Lawrence Welk but you can never hear the guitar in his music. Even Chet Atkins doesn't play "clean"!
 
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Spence

Joined Apr 23, 2010
49
Aren't all 20W guitar amps turned up to 40W or more so the output clips like crazy? They call it "fuzz".
Then the output transistors are simply on and off switches and are nice and cool.

I wonder what an electric "geetar" sounds like when played through a hifi system that is wideband and has no distortion. Has anybody ever tried it?
I've experimented with that and the guitar sounds all bass and treble, if you equalize it out, it starts to sound like an accoustic guitar but not so pleasant (Imagine 'wish you were here' by pink floyd) With a bit of stereo 'chorus' it starts to sound OK.
 

THE_RB

Joined Feb 11, 2008
5,438
"Wish You Were Here" has very little fuzz and sounds like acoustic guitars.
I like "Dark Side Of The Moon".
Actually "Wish you were here" was from the album "Wish you were here". ;)
But this is the first time I have ever agreed with your musical tastes, both Wish and the album before; Dark side of the moon were excellent.

Maybe there is hope for your musical tastes yet AG. :)

You might be surprised that much of the guitar especially David Gilmour's lead guitar used a lot of effects including fuzz and distortion etc. If done properly it sounds very nice.

Aren't all 20W guitar amps turned up to 40W or more so the output clips like crazy? They call it "fuzz". Then the output transistors are simply on and off switches and are nice and cool.
...
Not really, the guitar effects pedals that do the "fuzz" which is a smoothed clipping effect are BEFORE the amp, then the transistor amp is turned up to give the right volume. If you drive a transistor amp into amp clipping the result is absolutely terrible.

...
I wonder what an electric "geetar" sounds like when played through a hifi system that is wideband and has no distortion. Has anybody ever tried it?
Yep. My home recording setup has a nice low distortion stereo amp as the main amp and I play guitar clean through it and also with many types of effects including "heavy metal" type effects that I know you like so much.
 

#12

Joined Nov 30, 2010
18,222
Was that Peter Townsend of The Who?
and probably a few others..hehe...copycats!

Yay, Floyd. Every distortion effect ever invented and several they invented themselves.
You haven't been to Heaven until you've had a <snip> to Dark Side of the Moon!
 

Thread Starter

drk

Joined Mar 8, 2008
41
eh, I like wish you were here too! When the amp is done I'll (try to) play that song :)

Anyway, I've attached the layout I'm thinking of doing. Its more or less in scale, apart from the blue ones, those I still don't have the materials, or the board thought out.

Just one last question about the heatsinks, since I already have one heatsink (and going to buy the external one), can I just do like in the image, having both heatsinks? Should be ok right?


It livens the show and stops the noise.:)
They have other guitars though :)
 

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MrChips

Joined Oct 2, 2009
23,566
Don't know about you but I like to have my controls: GAIN, LOW, MID, HIGH, VOLUME
I have never understood the difference between VOLUME, LEVEL and GAIN.

I would also keep the preamp as far away as possible from the transformer and the power supply section.

Put two input jacks and gain controls.
Provide LINE OUT and HEADPHONES OUT.
 

THE_RB

Joined Feb 11, 2008
5,438
The heatsinks should be ok if they are fully screwed together metal plate to metal plate. That means you have to cut a big square hole in the back panel to join the heatsinks. Generally it works well to just cut a small square hole the size fo the amp IC, then screw the amp IC directly to the heatsink.

You don't really want a heatsink inside the amp case as you don't want to be dissipating (releasing) heat inside the case, you will get enough heat in there from the PSUs and from the front of the amp IC.
 

Thread Starter

drk

Joined Mar 8, 2008
41
[MrChips]
Yea, I guess that makes more sense for the way the circuit goes.

Volume and level seems to be the same thing (a potenciometer positioned in the end of the pre-amp -- before going to the power-amp). It controls the volume coming out of the preamp.

A gain is to control a gain stage of the preamp (ok, is sort of like a volume control, but with a different purpose).

I'll position the preamp to the far left, as possible.

hmm, headphones out I doubt I'll need, but a line out is a good idea, I'll add it.

[THE_RB]

Hey, you seem to be contradicting yourself, you say its ok to have both, but then that I really don't want a heatsink inside...

Something I thought (don't know if is a good idea), is that I could make some openings in the box, above the IC, so the air could pass through there.
 

THE_RB

Joined Feb 11, 2008
5,438
...
[THE_RB]
Hey, you seem to be contradicting yourself, you say its ok to have both, but then that I really don't want a heatsink inside...

Something I thought (don't know if is a good idea), is that I could make some openings in the box, above the IC, so the air could pass through there.
Not really contradicting, you can use the double heatsinks if you haver already done it that way, but the best way is a big external heasink with the IC on that (as I said above).

Some vent holes in the enclosure are ok and can help. Just watch for safety that nothing metal can poke in the hole and short things out. Professional amps have some fine screen glued over vent holes. :)
 
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